1. Technical Field
The invention relates to manhole cover supports, and in particular to a separate riser assembly placed within an existing manhole frame to raise the height of the manhole cover to compensate for added roadway pavement. More particularly, the invention relates to a riser assembly which is locked in position within the manhole frame preventing horizontal and vertical movement therein.
2. Background Information
Most underground facilities such as sanitary and storm sewers, utility conduits, and the like have manhole openings to provide access thereto. These manholes usually are located in the street or roadway and consists of an inverted, generally bell-shaped metal frame mounted on top of a brick or concrete base structure. This metal frame has an internal, horizontal ledge for supporting the manhole cover so that the top of the cover is level with the top of the frame and surrounding roadway pavement.
Problems arise in the resurfacing of the roadways in that a layer of pavement is placed on top of the existing pavement resulting in the manhole cover being below the top surface of the new pavement, causing a depression in the roadway. It is quite difficult and expensive to raise the existing manhole frame sufficiently to compensate for the added pavement. Thus, various devices and methods have been constructed and used which enable an existing manhole cover to be raised to the level of the new pavement surface without raising the existing manhole frame. Some examples of these devices and methods are shown in U.S. Pat. Nos. 1,517,871, 3,218,943, 3,773,428, 3,891,337, 4,097,171, 4,225,266, 4,302,126, 4,690,584, and 4,995,757.
Many of these prior art devices use a metal frame formed with a gap wherein the metal frame sits upon a generally horizontal surface of the manhole frame and includes a device for expanding the frame outwardly into abutting engagement with a upwardly outwardly extending conical surface formed on the manhole frame, which prior to the installation of the new pavement formed the seat for receiving the manhole cover thereon. However, in certain manhole frames, this conical surface forms an excessively large obtuse angle with the adjacent horizontal ledge resulting in the riser frame moving upwardly from its seated position within the frame when forced outwardly by the expansion device and subjected to various forces, making it difficult to retain the riser frame assembly within the manhole opening. The horizontally outwardly extending force component exerted by the expansion device is not sufficient to retain the riser frame within the manhole opening due to the excessive large taper or angle of the conical surface of the manhole frame. Furthermore, it has been found that sliding and shifting of the riser assembly can occur within the manhole opening providing an unwanted movement of the riser assembly, which could occur over time caused by thermal expansion of the ground or supporting structure or movement by snow plows or repetitive motion of vehicle tires moving over the manhole cover.
U.S. Pat. No. 4,690,584 shows one manner of solving this problem by the use of concrete reinforcing steel bars welded to the riser frame. The bars are bent outwardly beneath a retaining lip of the manhole frame to secure the riser assembly in position. Although this construction may provide a suitable solution, it requires a number of additional manufacturing steps such as welding the bars to the riser frame and then requiring the welded bars to be bent inwardly after placement into the manhole frame by repeatedly striking the bars with a hammer to bend the bars in position. However, the bent bars may not provide a secure lock with the adjacent manhole frame still resulting in some movement of the riser frame within the manhole frame causing rattling and other unwanted results. Also, if such bendable bars are used with riser assemblies which have an outward expansion device, it does not provide any assistance in overcoming the unwanted movement of the riser assembly due to the large angle of the manhole frame conical surface.
Thus, the need exists for a manhole assembly, and in particular, for a manhole riser assembly which enables the riser assembly to be securely retained both vertically and horizontally within the manhole frame in a simple and less expensive manner than heretofore achieved by prior riser assemblies.